The Bulls Have It

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IT'S A CENTRAL FACT OF THE LIVESTOCK INDUSTRY: steers are more valuable than cows, because mature male cattle carry more beef to sell. Now researchers have announced a technique that may allow breeders to choose the sex of calves -- at conception. Since eggs carry the female X chromosome, an animal's sex is determined by whether the fertilizing sperm carries another X or a male Y.

Scientists sort out X and Y carriers by tagging sperm with a fluorescent dye; under ultraviolet light, the X sperm glow brighter. Then the sperm are electrically charged -- positive for male, negative for female -- and a laser beam separates the two. Egg and appropriate sperm are mated in a glass dish, and the resulting embryo is implanted in a cow. The technique is about 90% reliable, which is pretty good odds for farmers to get more beef for the buck. Theoretically, a similar process could be developed for selecting human sex, though that ethical quandary does not appear imminent.